Summary of Significant Accounting Policies - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Utility Plant and Depreciation [Line Items]
Net	$ 10,628,138	$ 10,145,312
Construction work in progress	816,307	682,807
Palo Verde sale leaseback, net of accumulated depreciation	117,385	121,255
Intangible assets, net of accumulated amortization	123,975	119,755
Nuclear fuel, net of accumulated amortization	123,139	125,201
Total property, plant and equipment	11,808,944	11,194,330
Electric Service
Utility Plant and Depreciation [Line Items]
Generation	7,336,902	7,158,729
Transmission	2,494,744	2,247,309
Distribution	5,543,561	5,339,322
General plant	847,025	797,703
Plant in service and held for future use	16,222,232	15,543,063
Accumulated depreciation and amortization	(5,594,094)	(5,397,751)
Net	10,628,138	10,145,312
Construction work in progress	816,307	682,807
Palo Verde sale leaseback, net of accumulated depreciation	117,385	121,255
Intangible assets, net of accumulated amortization	123,975	119,755
Nuclear fuel, net of accumulated amortization	123,139	125,201
Total property, plant and equipment	$ 11,808,944	$ 11,194,330